There has been disclosed an internal combustion engine that uses CNG (compressed natural gas) that is difficult to ignite and light oil that is easy to ignite as a main fuel and a supplementary fuel respectively, and supplies both the fuels into a cylinder to burn the fuels during engine operation at substantially intermediate and high loads (e.g., see Patent Document 1). In this internal combustion engine, in the case where the aforementioned both the fuels are supplied into the cylinder, CNG as the main fuel, which is difficult to ignite, is burned by igniting light oil.